The present invention relates to a process for the dehydrogenation of light paraffins essentially having 3 to 5 carbon atoms per molecule and using an active catalyst for dehydrogenating the charge and which is characterized in that it is carried out without premixing the charge with hydrogen or diluent at the inlet of the reactor. It more particularly relates to the synthesis of isobutene, which is used for the preparation of MTBE (methyl tert.-butyl ether) with a view to improving the octane number in petrol.
It is of interest to vaporize aliphatic hydrocarbons with a low boiling point such as propanes, butanes, isobutanes, pentanes and isopentanes, which can be recovered following the extraction of the unsaturated products of C.sub.3, C.sub.4 and C.sub.5 fractions from vapor-phase cracking or catalytic cracking, as well as LPG's or field gases. This justifies the interest attached to the performance of conversion processes for such hydrocarbons, which have high performance characteristics, are selective and economic, while also contributing to hydrogen formation.
The olefinic hydrocarbon production reaction has already been described, more particularly in U.S. Pat. No. 4,381,417 and U.S. Pat. No. 4,381,418, where use is made of platinum-based, supported metal catalysts. These patents describe a regenerative process in which there is a hydrogen injection in parallel to the hydrocarbon injection at the inlet of the reactor. Different solutions aiming at optimizing the recycling of the hydrogen-rich phase under the standpoint of thermal exchanges have been described. One of the major disadvantages is the need to recycle all or part of the hydrogen produced, which complicates the process diagram and increases costs.
Dehydrogenation reactions are very fast and reversible. The conversion rates are limited by the thermodynamic equilibrium conditions. The high temperatures and low partial pressures of hydrogen very favorably displace the reaction toward the formation of olefinic compounds. However, these severe conditions are very favorable to the formation of coke, which leads to the deactivation of the dehydrogenation catalysts. This is why the article, "Oleflex: C2-C5 dehydrogenation Updated," by B. V. Vora, P. R. Pujado and R. F. Anderson, Energy Progress, Vol. 6, No. 3, pp. 171-176 (1986), states that in order to maintain their stability, platinum-based catalysts are generally used in the presence of a hydrogen recycle, such as is described in U.S. Pat. No. 3,998,900. Other diluents can also be used, such as, e.g., water vapor, methane or ethane. However, U.S. Pat. No. 4,962,266 describes a process, preferably without hydrogen in the charge, which uses a catalyst incorporating platinum and a zincosilicalite. This patent refers to the low stability of a catalyst including platinum and chlorinated alumina.